feat: pin sessions to workspace revisions

backend/src/workspaces/registry.ts

@@ -153,6 +153,17 @@ export class WorkspaceRegistry {
     }
   }
 
+  /** Read a retained immutable snapshot for a session pinned to a historical commit. */
+  async readPinned(id: string, commit: string): Promise<{ workspace: WorkspaceDescriptor; workspaceConfigPath: string }> {
+    const snapshotPath = this.snapshotPath(safeCommit(commit), id);
+    try {
+      const source = await readFile(snapshotPath, "utf8");
+      return { workspace: parseWorkspaceYaml(source), workspaceConfigPath: snapshotPath };
+    } catch (error) {
+      throw workspaceError(error);
+    }
+  }
+
   /**
    * Publish canonical YAML and derived public documentation as one optimistic Git revision.
    * The browser never provides paths or generated artifacts; those are derived server-side.